With the widespread use of portable information devices such as laptop computers and cellphones, there is a rapidly growing demand for electrochemical devices such as primary batteries, secondary batteries, and electric double layer capacitors used as power supplies for the devices. Especially, such electrochemical devices are required to be made smaller, more lightweight, and thinner and are expected to be made more reliable. They have recently been finding new applications in power supplies for hybrid electric vehicles and power supplies for energy storage in addition to power supplies for portable information devices. Therefore, they are required to be made much more reliable.
Generally, electrolytic solutions containing electrolyte salts dissolved in solvents are used in electrochemical devices. For this reason, such solutions may cause problems such as liquid leakage and, if they are nonaqueous electrolytic solutions, inflammation and ignition. The use of electrolytic solutions is a major factor contributing to low reliability of the electrochemical devices. Therefore, the problems can be solved by using solid electrolytes instead of electrolytic solutions. Especially, polymer electrolytes are a very promising material because they can easily be made into thin films and also have excellent mechanical properties and pliability.
From such a viewpoint, polymer electrolytes have been studied extensively for a long time. Many proposals have been made since it was reported for the first time that the complexation of a poly(ethylene oxide) polymer with a type of alkali metal salt allows ion conduction to be exhibited (see Non-Patent Document 1).
Patent Document 1 proposes a semisolid gel-state polymer electrolyte composed of polymethylmethacrylate, an electrolyte salt such as LiClO4 or LiBF4, and an organic solvent.
Patent Document 2 proposes an electrochemical generator using an all-solid-state polymer electrolyte that is a solid solution of an electrolyte salt in a polymer containing heteroatoms such as oxygen or nitrogen, and shows polymer materials such as poly(ethylene oxide) and polyamine.
Patent Document 3 proposes a gel-state polymer electrolyte composition containing an electrolyte salt dissolved in a mixture of a polymer having a relative permittivity of 4 or more and an organic solvent having a relative permittivity of 10 or more, and shows polymer materials meeting these requirements such as nitrocellulose, phenolic resin, polyvinylidene fluoride, polyacrylonitrile, and chlorosulfonated polyethylene.
Patent Document 4 discloses a lithium solid electrolyte battery using metallic lithium as the anode and a metal chalcogenide as the cathode, and shows polymer electrolytes using solid electrolytes such as vinylidene fluoride copolymer, polyvinyl chloride, polyvinyl acetate, and polyvinylpyrrolidone.
Patent Document 5 proposes an ion-conducting solid-body composition using a polymer material, and discloses that polysiloxane is an excellent polymer material.
Patent Document 6 discloses a hybrid ionic conductor using an oxyethylene(meta)acrylate polymer.
In addition, Patent Document 7 discloses an aliphatic epoxy resin-based ion-conducting crosslinked resin composition, and Patent Document 8 discloses a polyphosphazene-based polymer electrolyte. Patent Document 9 discloses an ion-conducting polymer composite consisting of polyalkylene carbonate, a metal salt and an organic solvent, and Patent Document 10 discloses a polymer solid electrolyte and a polymer solid electrolyte battery using polyurethane. Patent Document 11 discloses polyvinyl alcohol-based ion-conducting composition and the like.
There are two types of polymer materials that have been proposed for polymer electrolytes as mentioned above: all-solid-state polymer electrolytes including a polymer material and an electrolyte salt and gel-state polymer electrolytes formed by mixing a solvent with a polymer material and an electrolyte salt. The following major problem is left unresolved.
Specifically, all-solid-state polymer electrolytes have not yet achieved ion conduction that is practically satisfactory. In contrast, gel-state polymer electrolytes require the mixing of a large amount of a solvent for practical ion conduction. So, these types of polymer electrolytes are only better in reliability than conventional electrochemical devices using liquid electrolytes, and high reliability that polymer electrolytes have originally been expected to have has not yet been realized.
After that, in line with the commercialization of lithium-ion secondary batteries, the application of polymer electrolytes to lithium-ion secondary batteries was proposed (see Patent Document 12). This triggered more extensive studies of polymer electrolytes, leading to the commercialization of lithium-ion secondary batteries using gel-state polymer electrolytes. As mentioned earlier, however, large amounts of solvents are added to these gel-state polymer electrolytes, leading to failure to achieve the high reliability that polymer electrolytes have originally been expected to have. As a result, most of the lithium-ion secondary battery market involves the use of liquid electrolytes, and lithium-ion secondary batteries using gel-state polymer electrolytes have a very small share of the market.
To solve this problem, various types of polymer materials have since been studied. Patent Document 13 proposes an ion-conducting gel-state polymer electrolyte consisting of polymer A (1 to 40 wt %) having a carbonyl group, polyvinylidene fluoride polymer B (20 to 70 wt %), metal salt C (1 to 50 wt %), and organic solvent D (20 to 85 wt %). The document shows that preferable examples of polymer A having a carbonyl group are polyester, polycarbonate and polyester carbonate, and that other examples include polyamide, polypeptides, polyurethane, and polyketone. However, this system also contains a large amount of an organic solvent and does not necessarily meet the desired ion conduction.
In addition, Patent Document 14 proposes that polymers or copolymers of aromatic monomers having a functional group that acts as ionic ligands are used for all-solid-state polymer electrolytes. It discloses that a monomer having a ketonic carbonyl group can also be used as an example of monomers for the copolymer. However, all copolymers derived from this monomer have a low content of a carbonyl group and low ion conduction.
As mentioned above, all-solid-state polymer electrolytes have not yet provided practical properties at present. On the other hand, lithium-ion secondary batteries using gel-state polymer electrolytes have been put into practical use only in very limited applications for small consumer products, and the development of polymer electrolytes still has major problems at present.
Patent Document 1: JP-A-54-104541
Patent Document 2: JP-A-55-098480
Patent Document 3: JP-A-57-143356
Patent Document 4: JP-A-58-075779
Patent Document 5: JP-A-59-230058
Patent Document 6: JP-A-60-031555
Patent Document 7: JP-A-60-248724
Patent Document 8: JP-A-61-254626
Patent Document 9: JP-A-62-030147
Patent Document 10: JP-A-01-197974
Patent Document 11: JP-A-01-284508
Patent Document 12: JP-A-01-241767
Patent Document 13: JP-A-11-060870
Patent Document 14: JP-A-2006-012652
Non-Patent Document 1: P. V. Wright, Polymer, 14, 589 (1973)